The present invention relates to an image forming output apparatus such as a digital color copying machine, a digital color printer, and the like to which an electrophotographic process is applied.
In a copying machine and a printer to which an electrophotographic process is applied, an image is formed in the following manner: a cylindrical photoreceptor or a belt-shaped photoreceptor is rotated and electrostatic latent images are formed successively on the surface thereof; black toner and, in the case of a color image, other color toners are caused to adhere to the formed electrostatic latent images for development; and they are transferred onto a recording sheet, thus, the image is obtained. In this specification, the photoreceptor drum and a driving roller for the belt-shaped photoreceptor in the image output apparatus are referred to as a rotary body.
Accordingly, when the rotating speed of the photoreceptor is varied for some reasons, jittering or an uneven image is caused on the outputted image. These phenomena have remarkably appeared especially in the digital system electrophotographic technology employing scanning by means of a semiconductor laser for writing images on a photoreceptor. Fluctuation in rotating .speed of the photoreceptor has caused speed fluctuation of a writing system in the subsidiary scanning direction to create a slight difference in the distance between writing lines, contributing to the remarkable deteriorations of the image quality.
Conventionally, when designing the driving system for use in a copying machine or a printer, the main consideration has been put on how the objects driven by the driving device are appropriately located in the allowable space, while satisfying the values of the line speed or number of revolutions determined by the product specifications. That is, the following are main concerns: the method by which the driving power is transmitted from a driving power source to a driven object; and mechanical elements for power transmission.
Accordingly, in the case of jittering and rotational fluctuation caused in the finished product, the cause has been investigated, and the following remedies have been taken to change a bearing for a drive shaft of the photoreceptor to one made of sintered metal; to attach a flywheel on the drive shaft of the photoreceptor; to apply a brake, in which a spring is combined with a friction material on the rotary shaft of the photoreceptor drum; to improve the accuracy of a gear; or to use a helical gear with various kinds of helical angles.
However, in the development of a digital type image output apparatus, strict reproducibility of a one dot line written by a laser beam is required with an improvement of the apparatus performance, and accuracy required on the driving system has rapidly become strict. The accuracy required in this case is a level at which the uniformity of laser writing in the subsidiary scanning direction is assured in relation to the visual sensitivity of a visual organ. In order to accomplish this accuracy, it is the most essential technical theme to make the photoreceptor driving system highly accurate.
The main factors of the rotational fluctuation of the driving system have been found to be the following: the rotational fluctuation per one rotation of the rotating shaft of a motor is large, and absolute values of fluctuation components per one rotation of a gear and per one tooth of a gear are large; and fluctuation components and their higher harmonics components cause a resonance phenomenon in relation to the natural oscillation frequency of the driving system.
FIG. 12 shows the power spectrum of speed fluctuation in the driving system of conventional apparatuses. Fluctuation components of a gear according to the line speed proper to the apparatus are 176 Hz in the case of a gear directly coupled to the motor, 64 Hz in the case of a second shaft, and 25 Hz in the case of a gear directly coupled to a drum, and in this case, a higher harmonic component of 50 Hz is shown (not shown in a figure). Further, a component of a rotation of the gear directly coupled to the motor is 22 Hz, and its higher harmonic component of 44 Hz is shown.
FIG. 13 shows an example in which a transfer function for obtaining numerically the natural frequency of the driving system was measured. In this case, the measurement was conducted in the following way: an output of an impact excitation hammer, and an output of a piezoelectric type pick-up sensor, provided on one end of a photoreceptor drum so that the fluctuation of the acceleration in the rotation direction may be measured, were connected with a dual channel type FFT analyzer; and a Fourier spectrum ratio in each of them was obtained. From FIG. 13, the following can be found: a peak of the natural frequency of the present driving system is in the vicinity of 45 Hz; and areas of a high level transfer function are extended to cover the range of 30 to 60 Hz.
FIG. 14 shows the fluctuation component spectrum superimposed on the transfer function. In the driving system, it can be found from the drawing that a peak of the transfer function and the position of a frequency area where the fluctuation component and its second harmonics exist, are overlapped. That is, it has been found that the present driving system is one amplifying the fluctuation components (generating resonance).
Actually, when measurement data from three apparatuses each being equipped with the present driving system were investigated, the fluctuation of rotation of the photoreceptor showed values of 5 to 8%.
The present invention has been achieved in view of the foregoing, and its object is to provide a driving device for a rotary body wherein fluctuation rotation of a driving roller for a rotary body used in an image output apparatus is totally reduced from a low frequency region of several Hz--ten-odd Hz to a region of hundreds Hz--one thousand and several hundred Hz, and thereby an excellent color image free from unevenness, jitter, doubling and color shading can be obtained.